1. Field of the Invention
The present invention is directed to a passive infrared detector, and more particularly to a passive infrared detector for detecting the infrared radiation emanating from a person entering a monitored space to indicate the presence of the human in that space.
2. Description of the Prior Art
In the past, there have been provided a wide variety of passive infrared detectors comprising a single infrared sensor and an optical collector which gathers the infrared radiation from a space to be monitored and focuses the received radiation onto the sensor for providing an indication of the presence of a human being in that space. It is well known that the detector is desired to be pivotally supported on a base to be installed on a mounting surface such as a wall or ceiling for adjusting the angular orientation of the detector depending upon the geometry of the space being monitored. This is shown, for example, in U.S. Pat. Nos. 3,928,843 and 4,081,680. The former patent comprises a mirror assembly or optical collector with a sensor affixed thereto. The assembly is received within a housing which is mounted on a base by means of a swivel assembly so as to enable its angular position to be adjusted with respect to the base and therefore a wall or ceiling to which the base is installed. In the latter patent, a sensor is secured to an optical assembly in the form of a vat or trough-shaped reflector support which is provided with plural reflector elements for reflecting incident infrared radiation onto the sensor. The optical assembly combined with the sensor is pivotally supported by means of a bracket for the same purpose of adjusting its angular position. With the above prior detectors, when the optical collector is impaired or damaged while the sensor and associated electric circuitry of rather expensive construction are still operative, the entire assembly including the sensor and the electric circuitry have to be replaced together with the optical assembly. This imposes undue expenditure upon a user and may prevent the utilization of the detector in domestic and commercial applications. Further, when it is required to alter the covering range of space to be monitored or field of view, the entire detector assembly must be replaced with another type of detector having different optics suitable for receiving infrared radiation from the intended space or field of view, which also imposes undue expenditure as well as requires the cumbersome and inconvenient operation of removing the base of the existing detector from the mounting surface and then installing the new detector. This is disadvantageous in the sense that the sensor and its associated electric circuitry has to be replaced together with the associated optics or different types of optics. In this connection, none of the prior art detectors are found to disclose an arrangement in which the optical collector alone can be replaced while utilizing the sensor and its associated electric circuitry as the common components.